The present invention relates to an overhead projector for projecting an image of an original on a screen.
When images are to be projected onto a vertical screen without any angle of elevation using conventional overhead projectors, the deflection mirror a of the projector is positioned at an angle of inclination of 45.degree. as shown in FIG. 1. When images are to be projected at an angle of elevation (i.e., at an oblique angle above a horizontal plane; this angle will hereinafter be referred to sometimes as "oblique projection angle), the angle of inclination, 45.degree., of the mirror a is altered by .theta. as seen in FIG. 2. An image of the original c can then be projected on the vertical screen b at an angle of elevation of 2.theta.. Indicated at d is a projection lens for projecting and focusing the image on the screen.
However, the image forming plane e for the image projected at the angle 2.theta. is inclined at the angle 2.theta. with respect to the screen b. Consequently, the image projected on the screen b is distorted to a trapezoidal shape (trapezoidal distortion), is out of focus in the forward and rearward directions at the upper and lower portions of the screen b and is not uniformly in focus over the entire area of the screen.
The following three methods (FIGS. 3 to 5) are known for obviating these drawbacks.
With the method shown in FIG. 3, the screen b is inclined to extend along the image forming plane to eliminate trapezoidal distortion and loss of focus. However, this method requires a special screen, resulting in the disadvantage of rendering the device costly in its entirety.
In the case of the method of FIG. 4, the projection lens d is shifted toward the screen b, with the optical axis of the projection lens d intersecting an original support stage g at right angles therewith, whereby trapezoidal distortion and loss of focus can be obviated. Nevertheless, this method has the drawback of necessitating a projection lens of increased field angle thereby making the device complex.
FIG. 5 shows the third method, in which the projection lens d is inclined relative to the stage g to satisfy the Scheimpflug condition of: ##EQU1## where m: distance between the lens d and the stage g
f: focal distance of the lens d PA1 .alpha.: angle of inclination of the lens d PA1 .theta.: variation in the angle of inclination of the mirror a from 45.degree. (oblique projection angle=2.theta.)
This simple method is adapted to project focused images on the entire area of the screen. Various proposals resorting to this method have been made as disclosed, for example, in Examined Japanese Patent Publications SHO 47-26346 (conventional device 1) and SHO 48-31847 (conventional device 2), and Unexamined Japanese Patent Publication SHO 59-38433 (conventional device 3).
With conventional device 1, the inclination angle .theta. of the deflection mirror a is fixed, while the stage g is inclined with the focusing movement of the projection lens d.
With conventional device 2, the stage g is in a predetermined inclined position, and the angle of inclination .theta. of the deflection mirror a is variable with the focusing movement.
In the case of conventional device 3, the angle of inclination .alpha. of the projection lens d is adjustable independently.
Conventional devices 1, 2 and 3, nevertheless, have the problem that the oblique projection angle 2.theta. is not settable to a desired value by varying the inclination angle .theta. of the deflection mirror.
Although the angle .theta. of the mirror of conventional device 2 is variable with the focusing movement of the projection lens, loss of focus occurs if the mirror angle .theta. is optionally set.
With conventional devices 1 and 3, the inclination angle .theta. of the mirror is fixed or is not variable with the variation in the inclination angle .alpha. of the projection lens, so that when the mirror angle .theta. is altered, the adjustment of the inclination angle .alpha. of the lens and focus adjustment must be made independently.